Signal generator and frequency characteristic display method using signal generator

ABSTRACT

A signal generator includes inverse characteristic calculation means for calculating an inverse characteristic of a transfer function from an inverse characteristic of a frequency characteristic of a signal based on the transmission standard, inverse Fourier transform means for calculating impulse responses of a plurality of points by performing inverse Fourier transform on the inverse characteristic of the transfer function, impulse response cutout means for cutting out the points for a predetermined number of taps from the impulse response, frequency characteristic calculation means for calculating a frequency characteristic based on values of the points for the number of taps cut out from the impulse response, and display control means for displaying on a display screen, the frequency characteristic calculated by the frequency characteristic calculation means and an ideal frequency characteristic read from an S parameter file of a device under test.

TECHNICAL FIELD

The present invention relates to a signal generator for generating asignal with a desired loss value in order to test a device (DUT: deviceunder test) compliant with the digital signal transmission standard, anda frequency characteristic display method using the signal generator.

BACKGROUND ART

For example, as disclosed in the following Patent Document 1, an errorrate measuring apparatus is conventionally known as an apparatus whichtransmits a test signal including fixed data to a device under test suchas a photoelectric conversion component, and measures a bit error rate(BER) by comparing a measured signal which is input through the objectto be measured with a reference signal which is a standard on a bitbasis.

Meanwhile, as a device under test of this type of error rate measuringapparatus, in a device compliant with digital signal transmissionstandards such as PCIeGen 4.0, USB 3.0/3.1, Thunderbolt, or the like,for example, in order to evaluate the characteristics of a test boardfor each transmission standard, it is necessary to introduce an ISIcalibration channel which is a test fixture simulating a prescribedtransmission line loss, in input to a test board.

However, the amount of loss of the ISI calibration channel variesaccording to transmission standards. Therefore, the user needs toprepare a loss board of the ISI calibration channel conforming to thetransmission standard separately from the test board for eachtransmission standard.

RELATED ART DOCUMENT Patent Document

[Patent Document 1] JP-A-2007-274474

DISCLOSURE OF THE INVENTION Problem that the Invention is to Solve

Meanwhile, in a case of simulating the frequency characteristics of theISI calibration channel with the finite impulse response (FIR) filter,the number of taps of the FIR filter affects the deviation from theideal frequency characteristic, and as the number of taps decreases, theamount of deviation from the ideal frequency characteristic increases.Further, since steep frequency characteristics cannot be simulated witha FIR filter with small number of taps, the reproduction may beimpossible depending on the simulated frequency characteristics in somecases.

Thus, since the frequency characteristics are simulated with the FIRfilter, even though there is a limit to simulation, the index isinvisible to the user and cannot be visually recognized by the user.Therefore, the user is not able to directly determine whether thefrequency characteristic can be simulated correctly or not.

The present invention has been made in view of the above problems, andan object of the present invention is to provide a signal generatorcapable of visualizing the degree of reproduction of a waveform by a FIRfilter and a frequency characteristic display method using the signalgenerator.

Means for Solving the Problem In order to achieve the above purpose, asignal generator described in a first aspect of the present invention isa signal generator including:

inverse characteristic calculation means for, based on informationrepresenting a frequency characteristic which simulates a prescribedtransmission line loss, calculating an inverse characteristic of atransfer function from an inverse characteristic of the frequencycharacteristic, in order to test a device under test;

inverse Fourier transform means for calculating an impulse responsecomposed of a plurality of points, from the inverse characteristic ofthe frequency characteristic;

impulse response cutout means for cutting out the points for the numberof taps in a desired range with a reference to a peak of an amplitude ofthe impulse response calculated by the inverse Fourier transform means,from the impulse response;

tap coefficient calculation means for calculating tap coefficients forthe number of taps;

signal generation means for generating a signal with a loss whichsimulates the prescribed transmission line loss, by setting thecalculated tap coefficient in a finite impulse response (FIR) filter;

a display screen;

frequency characteristic calculation means for calculating a frequencycharacteristic based on values of the points for the number of taps inthe desired range cut out from the impulse response by the impulseresponse cutout means; and

display control means for identifying and displaying on the displayscreen, the frequency characteristic calculated by the frequencycharacteristic calculation means and an ideal frequency characteristicbased on information representing the simulated frequencycharacteristic.

In the signal generator described in a second aspect, the informationrepresenting the frequency characteristic of the device under test is anS parameter file.

In the signal generator described in a third aspect, the identifying anddisplaying is displaying the frequency characteristics so as to bedistinguishable by changing at least one of color classification, linetype, and line thickness, on a graph on the display screen, with ahorizontal axis as a frequency, and a vertical axis as an amplitude.

In the signal generator described in a fourth aspect, the displaycontrol means performs display control of the display means by addingthe inverse characteristic of the frequency characteristic calculated bythe frequency characteristic calculation means to the ideal frequencycharacteristic read from the information representing the frequencycharacteristic of the device under test.

A frequency characteristic display method using a signal generatorhaving a display screen described in a fifth aspect is a frequencycharacteristic display method using a signal generator, including

a step of, based on information representing a frequency characteristicwhich simulates a prescribed transmission line loss, calculating aninverse characteristic of a transfer function from an inversecharacteristic of the frequency characteristic, in order to test adevice under test;

a step of calculating an impulse response composed of a plurality ofpoints, from the inverse characteristic of the frequency characteristic;

a step of cutting out the points for the number of taps in a desiredrange with a reference to a peak of an amplitude of the impulseresponse, from the impulse response;

a step of calculating the frequency characteristic based on values ofthe points for the number of taps in the desired range cut out from theimpulse response

a step of calculating tap coefficients for the number of taps;

a step of generating a signal with a loss which simulates the prescribedtransmission line loss, by setting the calculated tap coefficient in afinite impulse response (FIR) filter; and

a step of identifying and displaying a frequency characteristiccalculated based on values of the points for the number of taps in thedesired range cut out from the impulse response and an ideal frequencycharacteristic based on information representing the simulated frequencycharacteristic, on the display screen.

In the frequency characteristic display method described in a sixthaspect, the information representing the frequency characteristic of thedevice under test is an S parameter file.

In the frequency characteristic display method described in a seventhaspect, the identifying and displaying is displaying the frequencycharacteristics so as to be distinguishable by changing at least one ofcolor classification, line type, and line thickness, on a graph on thedisplay screen, with a horizontal axis as a frequency, and a verticalaxis as an amplitude.

In the frequency characteristic display method described in an eighthaspect, the display means is controlled by adding an inversecharacteristic of the frequency characteristic calculated by thefrequency characteristic calculation means to an ideal frequencycharacteristic read from the information representing the frequencycharacteristic of the device under test.

ADVANTAGE OF THE INVENTION

According to the present invention, the degree of reproduction of awaveform by the FIR filter can be visualized, so the user views thevisualized degree and is able to directly determine whether thefrequency characteristic can be simulated correctly or not.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a schematic configuration of a signalgenerator according to the present invention.

FIG. 2 is a diagram showing a configuration example of a FIR filter usedin the present invention.

FIG. 3 is a flowchart of a frequency characteristic display method usingthe signal generator according to the present invention.

FIG. 4 is a diagram showing a display example in a state wherereproduction of a waveform by the FIR filter is possible.

FIG. 5 is a diagram showing a display example in a state wherereproduction of a waveform by the FIR filter is impossible.

FIG. 6 is a flowchart of a method of generating an ISI signal using thesignal generator according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, best modes for carrying out the present invention will bedescribed in detail with reference to the accompanying drawings.

The signal generator according to the present invention generates asignal (hereinafter referred to as an ISI signal) applied with an intersymbol interference (ISI) based on a desired loss value (the amount ofloss) based on the transmission standard of a digital signal, in orderto test a device compliant with the transmission standard of digitalsignals. Incidentally, the transmission standards of digital signals towhich the present invention is applied include PCIeGen 4.0, USB 3.0/3.1,Thunderbolt, and the like.

As shown in FIG. 1, in order to provide a platform enabling a user torecognize at a glance an ideal frequency characteristic and a frequencycharacteristic to be actually simulated, a signal generator 1 accordingto the present embodiment includes inverse characteristic calculationmeans 2, inverse Fourier transform means 3, impulse response cutoutmeans 4, tap coefficient calculation means 5, signal generation means 6,storage means 7, frequency characteristic calculation means 8, displaycontrol means 9, and display means 10.

The inverse characteristic calculation means 2 calculates a transferfunction and an inverse characteristic thereof, from the inversecharacteristic of the frequency characteristic of input data, with asignal based on the transmission standard of a digital signal and thefrequency characteristic of a device (device under test) to be simulatedactually as the input data.

The inverse Fourier transform means 3 performs an inverse Fouriertransform on the inverse characteristic of the transfer functioncalculated from the inverse characteristic of the frequencycharacteristic to be actually simulated by the inverse characteristiccalculation means 2 to calculate an impulse response composed of aplurality of points.

The impulse response cutout means 4 cuts out points for the number oftaps in a desired range (for example, 10 taps: 6 post 3 pre) from theimpulse response with reference to the peak of the amplitude of theimpulse response obtained by the inverse Fourier transform means 3.

The value (tap value) cut out from the impulse response is a filtercoefficient of a FIR filter 6 a (to be described later) for distortingthe input waveform to the test board and is used for obtaining the tapcoefficient.

The tap coefficient calculation means 5 calculates a tap coefficient byobtaining the gain/loss ratio therefrom with reference to a main tap ofthe FIR filter 6 a to be described later, by using the values of thepoints cut out from the impulse response by the impulse response cutoutmeans 4.

The signal generation means 6 includes the FIR filter 6 a, and sets thetap coefficient calculated by the tap coefficient calculation means 5 inthe FIR filter 6 a, and generates an ISI signal of frequencycharacteristics compliant with the desired transmission standard fordistorting the input wave to the test board.

In the case of, for example, ten taps, as shown in FIG. 2, the FIRfilter 6 a includes ten delay circuits 21 (21 a, 21 b, 21 c, 21 d, 21 e,21 f, 21 g, 21 h, 21 i, and 21 j) such as D-type flip flops, tenmultipliers 22 (22 a, 22 b, 22 c, 22 d, 22 e, 22 f, 22 g, 22 h, 22 i,and 22 j), and nine adders 23 (23 a, 23 b, 23 c, 23 d, 23 e, 23 f, 23 g,23 h, and 23 i). Ten delay circuits 21 a, 21 b, 21 c, 21 d, 21 e, 21 f,21 g, 21 h, 21 i, and 21 j are connected in series between the inputterminal 24 and the output terminal 25 to form ten taps.

Ten multipliers 22 a, 22 b, 22 c, 22 d, 22 e, 22 f, 22 g, 22 h, 22 i,and 22 j for multiplying the set tap coefficients are connected torespective taps of the FIR filter 6 a. The outputs of the stages beforeand after the ten multipliers 22 a, 22 b, 22 c, 22 d, 22 e, 22 f, 22 g,22 h, 22 i, and 22 j are connected to the corresponding stages of thenine adders 23 a, 23 b, 23 c, 23 d, 23 e, 23 f, 23 g, 23 h, and 23 i.Then, the FIR filter 6 a calculates and outputs the sum ofmultiplication results of the ten multipliers 22 a, 22 b, 22 c, 22 d, 22e, 22 f, 22 g, 22 h, 22 i, and 22 j.

The storage means 7 uses an S parameter file (S₂₁: transmissioncharacteristic) indicating a frequency characteristic of a device to besimulated of the ISI calibration channel as input data and stores an Sparameter file of the device.

The frequency characteristic calculation means 8 calculates a frequencycharacteristic by inverse Fourier transform from the tap value of theimpulse response cut out by the impulse response cutout means 4.

The display control means 9 reads an ideal frequency characteristic fromthe S parameter file (S₂₁: transmission characteristic) of the device tobe simulated of the ISI calibration channel stored in the storage means7, and controls the display means 10 such that the read ideal frequencycharacteristic is displayed on a graph (horizontal axis: frequency[GHz], and vertical axis: amplitude [dB]) on the display screen 10 a ofthe display means 10.

Further, the display control means 9 controls the display means 10 suchthat the frequency characteristic calculated by the frequencycharacteristic calculation means 8 is displayed on the graph on thedisplay screen 10 a of the display means 10 so as to be distinguishablefrom the ideal frequency characteristic.

The display means 10 is a display device such as a liquid crystaldisplay provided in the main body of the apparatus, for example, andunder the control of the display control means 9, displays the idealfrequency characteristic F1 read from the S parameter file and thefrequency characteristic F2 calculated by the frequency characteristiccalculation means 8 on a graph (horizontal axis: frequency [GHz], andvertical axis: amplitude [dB]) on the display screen 10 a so as to bedistinguishable from each other, for example, by changing colorclassification, line type, line thickness, or the like. For example, theideal frequency characteristic (DUT S-Parameter) F1 is identified anddisplayed by a solid line and the frequency characteristic (EmulateS-Parameter) F2 calculated by the frequency characteristic calculationmeans 8 is identified and displayed by a dotted line, on the graph onthe display screen 10 a of the display form shown in FIGS. 4 and 5.

In the present embodiment, the S parameter file of the device is storedin the storage means 7, but when simulating the ISI calibration channelwith the FIR filter 6 a, the S parameter file of a device to besimulated may be fetched from an external device, an external medium, orthe like.

Frequency Characteristic Display Method

Next, a frequency characteristic display method using the signalgenerator 1 configured as described above will be described withreference to FIGS. 3 to 5.

In a case of simulating the ISI calibration channel with the FIR filter6 a, the display control means 9 reads the S parameter file of a deviceto be simulated from the storage means 7, and reads an ideal frequencycharacteristic from the read S parameter file to display the idealfrequency characteristic on the graph on the display screen 10 a of thedisplay means 10 (ST1). For example, the ideal frequency characteristic(solid line) F1 is displayed on the graph on the display screen 10 a ofeach of the display forms shown in FIGS. 4 and 5. The S parameter fileof the device stored in the storage means 7 can be read out by pressing“Open” composed of soft keys on the display screens 10 a of FIG. 4 andFIG. 5, for example.

Next, the inverse characteristic calculation means 2 calculates thetransfer function and its inverse characteristic from the inversecharacteristic of the frequency characteristic simulated by the FIRfilter 6 a (ST2). Then, the inverse Fourier transform means 3 performsan inverse Fourier transform on the inverse characteristic of thetransfer function calculated by the inverse characteristic calculationmeans 2 to calculate an impulse response of a plurality of points (ST3).

Next, the impulse response cutout means 4 cuts out points for apredetermined number of taps from the impulse response calculated by theinverse Fourier transform means 3 (ST4). Subsequently, the frequencycharacteristic calculation means 8 calculates a frequency characteristicby inverse Fourier transform from the values of points for the number oftaps cut out from the impulse response by the impulse response cutoutmeans 4 (ST5).

Then, the display control means 9 displays the frequency characteristiccalculated by the frequency characteristic calculation means 8 bysuperimposing the frequency characteristic on the graph on the displayscreen 10 a of the display means 10 so as to be distinguishable from theideal frequency characteristic (ST6). For example, the frequencycharacteristic (dotted line) F2 calculated by the frequencycharacteristic calculation means 8 and the ideal frequencycharacteristic (solid line) F1 are superimposed so as to bedistinguishable from each other and displayed on the graph on thedisplay screen 10 a of the display form shown in FIGS. 4 and 5.

Here, FIG. 4 shows a display example in a state where reproduction of awaveform by the FIR filter 6 a is possible, and FIG. 5 shows a displayexample in a state where reproduction of a waveform by the FIR filter 6a is impossible.

In a case where the frequency characteristic can be sufficientlysimulated for the number of taps of the FIR filter 6 a, as shown in FIG.4, the frequency characteristic (Emulate S-Parameter: dotted line) F2calculated by the frequency characteristic calculation means 8 and theideal frequency characteristic (DUT S-Parameter: solid line) F1 areclose to each other.

On the other hand, for example, even when the amount of loss at theNyquist frequency is the same as in FIG. 4, in a case where the changein frequency characteristics is steep, as shown in FIG. 5, the frequencycharacteristic (Emulate S-Parameter: dotted line) F2 calculated by thefrequency characteristic calculation means 8 and the ideal frequencycharacteristic (DUT S-Parameter: solid line) F1 do not indicate valuesclose to each other, and the waveform by the FIR filter 6 a cannot bereproduced.

In addition to this, the reproducing of the characteristics of a devicehaving a steep frequency characteristic such as LPF is impossible, butthe ideal frequency characteristic and the actually simulated frequencycharacteristic are displayed so as to be distinguishable as in thepresent embodiment, and are visible at a glance.

ISI Signal Generation Method

Next, a method of generating an ISI signal by the signal generator 1will be described with reference to FIG. 6.

The inverse characteristic calculation means 2 calculates the transferfunction and its inverse characteristic from the inverse characteristicof the frequency characteristic of input data in a state wherereproduction of a waveform by the FIR filter 6 a is possible, by theabove-described frequency characteristic display method (ST11).

Next, the inverse Fourier transform means 3 performs an inverse Fouriertransform on the inverse characteristic of the transfer functioncalculated by the inverse characteristic calculation means 2 tocalculate an impulse response of a plurality of points (ST12).

Next, the impulse response cutout means 4 cuts out points for apredetermined number of taps from the impulse response calculated by theinverse Fourier transform means 3 (ST13).

Next, the tap coefficient calculation means 5 calculates a tapcoefficient by the gain/loss ratio therefrom with reference to the tapof Main of the FIR filter 6 a, by using the values of the points cut outfrom the impulse response by the impulse response cutout means 4 (ST14).

Then, the signal generation means 6 sets the tap coefficients calculatedby the tap coefficient calculation means 5 in the FIR filter 6 a, andgenerates an ISI signal of a desired frequency characteristic (ST15).

As described above, according to the present embodiment, an idealfrequency characteristic is read from the S parameter file of a deviceto be simulated, and displayed on a graph. Further, a frequencycharacteristic is calculated by inverse Fourier transformation from thetap value of the impulse response in a case of simulating an idealfrequency characteristic by the FIR filter is calculated, and thecalculated frequency characteristic and the ideal frequencycharacteristic are displayed so as to be distinguishable from eachother. This makes it possible to visualize the degree of reproduction ofa waveform by the FIR filter, and it is possible to easily check whatextent of the ideal frequency characteristic the actually simulatedfrequency characteristic is simulated, by comparing the ideal frequencycharacteristic with the actually simulated frequency characteristic onthe display screen.

Meanwhile, the display control means 9 performs display control of thedisplay means 10 by adding the inverse characteristic of the frequencycharacteristic calculated by the frequency characteristic calculationmeans 8 to the ideal frequency characteristic read from the S parameterfile of the device stored in the storage means 7. In this case, on thedisplay screen of the display means 10, the difference in amplitudebetween the frequency characteristic calculated by the frequencycharacteristic calculation means 8 and the ideal frequencycharacteristic is displayed. Thus, the user can determine whether or notthe difference in amplitude falls within an allowable range, and whetheror not the loss can be corrected correctly, by seeing the difference inamplitude on the display screen.

Although the best mode of the signal generator and the frequencycharacteristic display method using the signal generator has beendescribed above, the present invention is not limited by the descriptionand drawings according to this mode. In other words, it is a matter ofcourse that other modes, examples, operation techniques and the likemade by those skilled in the art based on this mode are all included inthe scope of the present invention.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

1 Signal Generator

2 Inverse Characteristic Calculation Means

3 Inverse Fourier Transform Means

4 Impulse Response Cutout Means

5 Tap Coefficient Calculating Means

6 Signal Generation Means

6 a FIR Filter

7 Storage Means

8 Frequency Characteristic Calculation Means

9 Display Control Means

10 Display Means

10 a Display Screen

21 (21 a, 21 b, 21 c, 21 d, 21 e, 21 f, 21 g, 21 h, 21 i, 21 j) DelayCircuit

22 (22 a, 22 b, 22 c, 22 d, 22 e, 22 f, 22 g, 22 h, 22 i, 22 j)Multiplier

23 (23 a, 23 b, 23 c, 23 d, 23 e, 23 f, 23 g, 23 h, 23 i, 23 j)

Adder

24 Input Terminal

25 Output Terminal

F1 Ideal Frequency Characteristic (Solid Line)

F2 Simulated Frequency Characteristic (Dotted Line)

What is claimed is:
 1. A signal generator comprising: inversecharacteristic calculation means for, based on information representinga frequency characteristic which simulates a prescribed transmissionline loss, calculating an inverse characteristic of a transfer functionfrom an inverse characteristic of the frequency characteristic; inverseFourier transform means for calculating an impulse response composed ofa plurality of points, from the inverse characteristic of the frequencycharacteristic; impulse response cutout means for cutting out the pointsfor the number of taps in a desired range with a reference to a peak ofan amplitude of the impulse response calculated by the inverse Fouriertransform means, from the impulse response; tap coefficient calculationmeans for calculating tap coefficients for the number of taps; signalgeneration means for generating a signal with a loss which simulates theprescribed transmission line loss, by setting the calculated tapcoefficient in a finite impulse response (FIR) filter; a display screen;frequency characteristic calculation means for calculating a frequencycharacteristic based on values of the points for the number of taps inthe desired range cut out from the impulse response by the impulseresponse cutout means; and display control means for displaying on thedisplay screen, the frequency characteristic calculated by the frequencycharacteristic calculation means and an ideal frequency characteristicbased on information representing the simulated frequencycharacteristic.
 2. The signal generator according to claim 1, whereinthe information representing the frequency characteristic of theprescribed transmission line loss is an S parameter file.
 3. The signalgenerator according to claim 1, wherein the identifying and displayingis displaying the frequency characteristics so as to be distinguishableby changing at least one of color classification, line type, and linethickness, on a graph on the display screen, with a horizontal axis as afrequency, and a vertical axis as an amplitude.
 4. The signal generatoraccording to claim 1, wherein the display control means performs displaycontrol of the display means by adding the inverse characteristic of thefrequency characteristic calculated by the frequency characteristiccalculation means to the ideal frequency characteristic read from theinformation representing the frequency characteristic of the prescribedtransmission line loss.
 5. A frequency characteristic display methodusing a signal generator having a display screen, comprising: a step of,based on information representing a frequency characteristic whichsimulates a prescribed transmission line loss, calculating an inversecharacteristic of a transfer function from an inverse characteristic ofthe frequency characteristic; a step of calculating an impulse responsecomposed of a plurality of points, from the inverse characteristic ofthe frequency characteristic; a step of cutting out the points for thenumber of taps in a desired range with a reference to a peak of anamplitude of the impulse response, from the impulse response; a step ofcalculating the frequency characteristic based on values of the pointsfor the number of taps in the desired range cut out from the impulseresponse; a step of calculating tap coefficients for the number of taps;a step of generating a signal with a loss which simulates the prescribedtransmission line loss, by setting the calculated tap coefficient in afinite impulse response (FIR) filter; and a step of displaying afrequency characteristic calculated based on values of the points forthe number of taps in the desired range cut out from the impulseresponse and an ideal frequency characteristic based on informationrepresenting the simulated frequency characteristic, on the displayscreen.
 6. The frequency characteristic display method according toclaim 5, wherein the information representing the frequencycharacteristic of the prescribed transmission line loss is an Sparameter file.
 7. The frequency characteristic display method accordingto claim 5, wherein the identifying and displaying is displaying thefrequency characteristics so as to be distinguishable by changing atleast one of color classification, line type, and line thickness, on agraph on the display screen, with a horizontal axis as a frequency, anda vertical axis as an amplitude.
 8. The frequency characteristic displaymethod according to claim 5, wherein the display means is controlled byadding an inverse characteristic of the frequency characteristiccalculated by the frequency characteristic calculation means to an idealfrequency characteristic read from the information representing thefrequency characteristic of the prescribed transmission line loss.